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The ArmOID and EpochOID attributes may only be present when the StudyEventGroupDef represents an upper-level study design building block, i.e. describing a "study cell", which is the crossing of an arm with an epoch, as e.g. defined in the SDTM-IG.
In such a case, the value of ArmOID must be equal to the value of the OID of an Arm (child element in StudyDesignof StudyStructure). The value of EpochOID must be equal to the value of the OID of an Epoch (child element in of StudyStructure).
The ArmOID and EpochOID must be absent when the StudyEventGroupDef is referenced from another StudyEventGroupDef.
Whereas StudyEventGroupDefs having both the ArmOID and EpochOID populated represent a study design "study cell", as e.g. defined in the SDTM-IG , StudyEventGroupDefs without ArmOID and EpochOID being populated, but referenced by a StudyEventGroupDef that has both ArmOID and EpochOID populated, will typically represent "study elements", as defined in the SDTM-IG. Sub-elements can then be defined by creating additional StudyEventGroupDefs that are referenced by StudyEventGroupDefs representing study elements.
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